1. Field of the Invention
The present invention relates to a hydrodynamic bearing, more particularly to a hydrodynamic bearing for use in spindle motors, drum motors, or the like in data storage devices.
2. Description of the Related Art
Spindle motors and drum motors in data storage devices, including hard disk drives (HDD) and magnetic tape storage devices for example, are required to have highly accurate rotational performance and low power requirements. To meet such performance and power requirements, efforts have been made to employ hydrodynamic bearings in such spindle motors and drum motors. However, the hydrodynamic bearings suffer certain inherent drawbacks. Specifically, the hydrodynamic bearings have sliding surfaces that are held in contact with each other when they start and stop rotating. Depending on the material which the sliding surfaces are made of, how they are machined, and the accuracy with which they are assembled, the sliding surfaces may wear very rapidly owing to frictional contact therebetween, thus posing a durability problem.
One solution to the above problem is to make at least the sliding surfaces out of a ceramic material which is highly durable and machinable with high dimensional accuracy. In some cases, all the parts of a hydrodynamic bearing are made of a ceramic material, and then sliding surfaces are ground to a flat finish. Alternatively, all the parts of a hydrodynamic bearing are made of a metallic material such as stainless steel, aluminum, or the like, then sliding surfaces are ground to a flat finish and are coated with a ceramic material such as silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), alumina (Al.sub.2 O.sub.3), or the like.
However, even ceramic-coated hydrodynamic bearings fail to satisfy a desired level of durability when they are frequently started and stopped, and are hence not practical for use in data storage devices. Ceramic materials have such a property that when sliding surfaces are pressed against each other under a pressure in excess of a certain pressure level, the coefficient of friction abruptly increases, resulting in accelerating the wear on the sliding surfaces. This property of the ceramic materials is responsible for the above durability problem of the hydrodynamic bearings with ceramic sliding surfaces or ceramic coating films.
Attempts have been made to improve the wear resistance of the sliding surfaces under high pressures. One of the most simple ways is to coat a thin film of liquid lubricant such as oil or grease on the sliding surfaces.
When such a liquid lubricant is applied to the sliding surfaces, a lubricant film having a thickness ranging from 0.01 to several microns is formed on each of the sliding surfaces. However, the adhesion between the lubricant film and the sliding surfaces is so weak that the liquid lubricant tends to separate and scatter around when the bearing is rotated. Consequently, when such a hydrodynamic bearing is employed in a hard disk drive, the scattered liquid lubricant is apt to adversely affect the magnetic medium. Where the hydrodynamic bearing is incorporated in a drum motor, the scattered liquid lubricant is liable to become attached to the drum or heads, adversely affecting their performance. Thinning out the lubricant film in an effort to minimize the scattering of the liquid lubricant results in displacement of the liquid lubricant or shearing of the lubricant film when the bearing is repeatedly started and stopped. When this happens, the sliding surfaces of the bearing are brought into direct contact with each other, and their direct contact causes quick wear on the sliding surfaces.
To solve the above problems, there have heretofore been two proposals for improving the performance of hydrodynamic bearings as described below.
According to the first proposal, the sliding surfaces of a hydrodynamic bearing are coated with a self-lubricating solid lubricant such as molybdenum disulfide or graphite by sputtering or the like. The coated solid lubricant film is bonded to the sliding surfaces more strongly than the liquid lubricant, and hence will not be scattered around while the bearing is rotating. Under relatively low pressures, the coated solid lubricant film has a low coefficient of friction, and is more durable than the liquid lubricant film.
The second proposal, Japanese laid-open patent publication No. 64-65322 discloses a hydrodynamic bearing having rotatable confronting surfaces which slide against relative to each other. At least one of the sliding surfaces is coated with a lubricating film made of one or both of organopoly-siloxane and a fluoropolymer having a functional group. The lubricating film with a functional group is strongly bonded to the bearing surface, and hence is prevented from being moved or scattered during rotation of the bearing. Therefore, the sliding surfaces are highly resistant to wear.
In the first proposal, when the pressure applied to the sliding surfaces exceeds a certain pressure level, the nonpolar solid lubricant film on both sliding surfaces is removed by sliding contact with each other. Once the continuity of the lubricant film is broken, it cannot be recovered by itself. When the lubricant film is broken, the coefficient of friction of the sliding surfaces are abruptly increased, resulting in an increase in the frictional torque which is applied when the bearing starts rotating. After repeated starting and stopping of rotation of the bearing, the motor which incorporates the bearing may possibly be unable to start to rotate when energized.
For the bearing of the second proposal, a sliding test was conducted on a bearing whose sliding surfaces were coated with a lubricating film according to the disclosure. As long as the sliding surfaces were pressed against each other under a relatively low pressure, their coefficient of friction was low and remained unchanged for a long period of operation and provided better results than the nonpolar solid lubricant film of the first proposal. When the sliding surfaces were pressed against each other under a pressure in excess of about 50 gf/cm.sup.2, the lubricating film was broken and the coefficient of friction was abruptly increased.
With the thickness of the lubricating film exceeding about 60 .ANG., the lubricating film had a meniscus effect or underwent "stiction" caused probably by chemically activated adsorption, thus increasing the static frictional torque to the point where the motor was unable to start. When the lubricating film was too thin, it was broken while the bearing was repeatedly started and stopped under a relatively low contact pressure applied. Thus, the sliding surfaces themselves came into contact and were rapidly worn down.
The lubricating film according to the second proposal showed best results when its thickness ranged from about 30 to 50 .ANG.. However, it is highly difficult to keep the lubricating film thickness constant, making it impossible to mass-produce the bearing efficiently.
The hydrodynamic bearing generally includes a thrust hydrodynamic bearing assembly having flat sliding surfaces and a radial hydrodynamic bearing assembly having cylindrical sliding surfaces. These sliding surfaces are in contact with each other at the time of starting or stopping the bearing. In these cases, the cylindrical sliding surfaces of the radial hydrodynamic bearing assembly are held substantially in line-to-line contact with each other, and the flat sliding surfaces of the thrust hydrodynamic bearing assembly are held in plane-to-plane contact with each other. These sliding surfaces are subjected to a pressure imposed by the weight of a rotatable body that is supported by the bearing. Insofar as the bearing supports the same rotatable body, the cylindrical sliding surfaces of the radial hydrodynamic bearing assembly which are held substantially in line-to-line contact with each other undergo a much higher pressure than the flat sliding surfaces of the thrust hydrodynamic bearing assembly which are held in plane-to-plane contact with each other.
For example, when the hydrodynamic bearing is incorporated in a motor that has its axis extending vertically, the rotatable body of the motor is supported substantially by the thrust hydrodynamic bearing assembly. When the hydrodynamic bearing is incorporated in a motor that has its axis extending horizontally, the rotatable body of the motor is supported substantially by the radial hydrodynamic bearing assembly. Therefore, while the motor repeatedly starts and stops rotating, the lubricating film on the sliding surfaces of the bearing deteriorates to a much greater degree when the motor axis lies horizontally than when the motor axis lies vertically.
A contact-start-and-stop (CSS) test was conducted on a hydrodynamic bearing incorporated in a spindle motor for use in a hard disk drive and having a lubricating film according to the second proposal. In the test, the bearing was started and stopped about 40,000 times reliably when the motor axis was extending vertically, but failed to start rotating after it was started and stopped about 500 times when the motor axis was extending horizontally. This indicates that as long as the motor axis extends horizontally, there is a practical problem with respect to the wear resistance of the sliding surfaces of the radial hydrodynamic bearing assembly even though the sliding surfaces are coated with a lubricating film.
The sliding surfaces have grooves for developing a hydrodynamic pressure, the grooves having a depth ranging from several microns to several tens of microns. These sliding surfaces and their components may be made of ceramics, and the sliding surfaces may be finished smoothly by grinding and other methods. Or, the sliding components may be made of metallic materials such as stainless steel or aluminum, and after making the sliding surfaces smooth, coatings such as silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4) or alumina (Al.sub.2 O.sub.3) may be applied. The grooves in these sliding surfaces may be formed by thermal removal techniques such as laser processing, or by material removal technique such as ion etching or shot blasting.
When the grooves are formed in the ceramic sliding surfaces or in the ceramic coatings of the sliding surfaces, many microcracks are developed in the groove surfaces due to applied shocks or vibrations. Since the ceramic materials are brittle, each time the sliding surfaces contact each other when the bearing is started or stopped, the microcracks grow due to applied shocks until finally the ceramic sliding surfaces or the ceramic coatings suffer intergranular fracture.
Ceramic particles produced and separated from the ceramic sliding surfaces or the ceramic coatings by the intergranular fracture are about several microns across, and partly accumulated in the grooves and the gap between the sliding surfaces and partly discharged out of the bearing, The ceramic particles retained in the gap between the sliding surfaces cause abnormal wear on the sliding surfaces, shortening the service life of the bearing. The ceramic particles discharged out of the bearing contaminate the external environment. If the bearing is incorporated in a hard disk drive, then the discharged ceramic particles are likely to bring about fatal troubles such as a head crash or the like.